1. Field of the Invention:
The present invention relates to a process for the polycondensation of polysiloxanes comprised of hydroxysilyl functions, and, more especially, to the catalysis of such polycondensation employing, as a co-catalyst therefor, an alkali or alkaline earth metal in conjunction with a certain polyheteromacropolycyclic compound.
2. Description of the Prior Art:
It is well known to this art that the silanols, silanediols and polysiloxanediols can be polycondensed in the presence of varied compounds which accelerate the polycondensation by acting as a dehydrating agent or catalyst (compare, for example, the treatise by Walter NOLL: Chemistry and Technology of Silicones, 1968 edition, pages 211 to 218).
Dehydrating agents of the aforesaid type which are representative are, on the one hand, acidic compounds, such as sulfuric and phosphoric acids and acid chlorides, and, on the other hand, non-acidic compounds, such as isocyanates, boric acid esters and the like. All of such agents must be used in stoichiometric proportions.
Catalysts of the aforesaid type which are representative are hydrogen halide acids, basic catalysts, such as alkali metal hydroxides, and amines, such as triethylamine. Finally, the polycondensation reaction can also be activated by conducting same in the presence of organometallic compounds of metals such as lead, tin, zirconium, aluminum, calcium, sodium, potassium and the like.
Moreover, these various catalysts are also reputed to promote equilibration among the siloxane bonds by opening the latter, with the subsequent polymerization of such opened bonds, and hence with the formation of volatile cyclic compounds [Journal of Polymer Science, 59, 259-269 (1962)]. It then appears to be obligatory to effect a devolatilization treatment, after the polycondensation, in order to ultimately obtain organosilicon elastomers possessing satisfactory properties.
French Pat. No. 2,168,439 then recommended the use of aluminum as a polycondensation catalyst, the aluminum optionally being coated with an oxide layer. A catalyst of this type, according to said French Pat. No. 2,168,439, does not give rise to transposition reactions among the siloxane bonds, but the polycondensation kinetics remain slow. It should, moreover, also be noted that the customary basic or acid catalysts as above outlined enable attainment of but a relatively slow rate of polycondensation.